There could be many possibilities. Some plastic checkvalves use a flexible rubber gate and hinge. After a fairly short time period, the rubber hinge hardens and/or the disc distorts, so it either cannot close, or seal if it does close. In addition, ALL backwater valves will fail if ANYTHING lodges in the opening so it cannot close tightly. You should only count on a backwater valve slowing the flooding down, and have a backup plan to deal with the water that does get by it. When I was in the Chicago area, we installed combination backwater valve and pump systems to take care of both problems.